"Rock climbing" or "bouldering" on buildings.
Usually done with no rope, perhaps a crash pad and climbing shoes. Bridge supports, retaining walls, strangely shaped buildings and modern art (where there are no security guards) are popular buildering spots.

When rock climbers are stuck in a city with no rock to be found they go buildering.

like normal rock climbing, buildering takes place in an urban setting on man-made objects such as buildings. usually only practiced by those crazy rock climbers, the point is to spot a very climb-able looking surface and attempt to scale it in sneakers with no other equipment.

"so i was walking around my town today, and the church looked so climbable so i started buildering"